Screen cylinders have long been used in pulp and papermaking applications to screen fiber in a stock slurry. Screen cylinders typically consist of a perforate cylinder received in the housing of a screening apparatus. Stock flows through slots or holes in the sidewall of the screen cylinder where it eventually ends up being processed into pulp, paper or another fiber product. Fiber in the stock selectively passes through the slots or holes in an accepts side of the screen cylinder and exits out a discharge side of the screen cylinder.
In the past, attempts have been made to provide a screen cylinder that included a replaceable perforate screen media supported by a frame. One such attempt is disclosed in U.S. Pat. No. 5,200,072. The '072 patent discloses a screen cylinder having a perforate backing plate that is heat shrink fit over a perforate screening plate. One problem with the screen cylinder disclosed in this patent is that it requires heating up the backing plate to expand it so that the screening plate can be inserted or removed. This is time consuming, labor intensive, and, as a result, costly.
Another problem with this arrangement is that the screen plate is prone to premature failure during operation because rotation of a foil of a rotor of the screening apparatus disposed inside the screen cylinder tends to pull the screen plate away from the backing plate adjacent the foil as it rotates. Since the backing plate does not engage the screen plate in a manner that restrains radial movement of the screen plate away from the backing plate, the screen plate deflects radially away from the backing plate each time the rotor foil rotates. As a result, repeated stress cycling of the screen plate created by negative pressure pulses that follow the rotor foil can cause the screen plate to crack and prematurely fail.
Additionally, because the backing plate does not provide radial support to the screen plate in both radial directions, the stress experienced by the screen plate is not uniform. This creates stress concentrations in the screen plate as a result of the screen plate being pulled away from the backing plate that also can prematurely accelerate failure. As a result of these deficiencies, this arrangement is believed to be limited to lower consistency screening applications. These deficiencies also limit the size and thickness of the screen plate and backing plate, which in turn prevents its application to pulp screening apparatuses having tight packaging requirements.
Wedgewire screen media have been used in pulp screening applications for many years. While wedgewire screen cylinders are cheaper to make than conventional screen cylinders, they tend to fail sooner because they are not as strong. One solution in the past has been to weld reinforcement rings to clips that retain the wires of the wedgewire screen media to increase strength. While this has worked to a limited degree, the weld that attaches each ring to the retainer clip creates a heat-affected zone in part of the clip. This heat-affected zone makes the clip more brittle and less able to withstand the repeated stress cycling that all screen cylinders typically encounter. As a result, wedgewire screen media can prematurely fail.
Another problem with wedgewire screen media is that the wire receiving slots in the retainer clips that hold the screening wires tend to be locations where cracks are more likely to form. These slots have end walls with corners that are relatively sharp, creating undesirable stress risers. As a result, cracks are more likely to form in the slots at the corners. Once formed, they are likely to quickly propagate through the clip causing screen media failure. Once formed, these cracks also tend to propagate through reinforcing rings, which then renders such rings less effective.
What is needed is a screen cylinder assembly that uses a frame and a replaceable screen media that can be of wedgewire construction that is longer lasting and more reliable. What is also needed is a frame and wedgewire screen media of improved construction.